1. Field of the Invention
The present invention relates to a photosensitive resin composition for forming a laser engravable printing element. More particularly, the present invention is concerned with a photosensitive resin composition for forming a laser engravable printing element, comprising: (a) a resin which is a plastomer at 20° C., wherein the resin has a number average molecular weight of from 1,000 to 100,000 and has a polymerizable unsaturated group in an amount such that the average number of the polymerizable unsaturated group per molecule is at least 0.7, (b) an organic compound having a number average molecular weight of less than 1,000 and having at least one polymerizable unsaturated group per molecule, and (c) an inorganic porous material. By the use of the photosensitive resin composition of the present invention, it becomes possible to produce a printing element which not only has high uniformity in thickness and high dimensional precision, but also generates only a small amount of debris during the laser engraving of the printing element. Further, the produced printing element is advantageous in that a precise image can be formed on the printing element by laser engraving and the resultant image-bearing printing plate has small surface tack. Further, the present invention is also concerned with a laser engravable printing element formed from the photosensitive resin composition of the present invention.
2. Prior Art
The flexographic printing method is used in the production of packaging materials (such as a cardboard, a paperware, a paper bag and a flexible packaging film) and materials for construction and furnishing (such as a wall paper and an ornamental board) and also used for printing labels. Such flexographic printing method has been increasing its importance among other printing methods. A photosensitive resin is generally employed for producing a flexographic printing plate, and the production of a flexographic printing plate using a photosensitive resin has conventionally been performed by the following method. A photo-mask bearing a pattern is placed on a liquid resin or a solid resin sheet (obtained by molding a resin into a sheet), and the resultant masked resin is imagewise exposed to light, to thereby crosslink the exposed portions of the resin, followed by developing treatment in which the unexposed portions of the resin (i.e., uncrosslinked resin portions) are washed away with a developing liquid. Recently, the so-called “flexo CTP (Computer to Plate) method” has been developed. In this method, a thin, light absorption layer called “black layer” is formed on the surface of a photosensitive resin plate, and the resultant resin plate is irradiated with laser to ablate (evaporate) desired portions of the black layer to form a mask bearing an image (formed by the unablated portions of the black layer) on the resin plate directly without separately preparing a mask. Subsequently, the resultant resin plate is imagewise exposed to light through the mask, to thereby crosslink the exposed portions of the resin, followed by developing treatment in which the unexposed portions of the resin (i.e., uncrosslinked resin portions) are washed away with a developing liquid. Since the efficiency for producing the printing plates has been improved by this method, its use is beginning to expand in wide variety of fields. However, this method also requires a developing treatment as in the case of other methods and, hence, the improvement in the efficiency for producing the printing plates is limited. Therefore, it has been desired to develop a method for forming a relief pattern directly on a printing element by using a laser without a need for a developing treatment.
As an example of a method for producing a printing plate by directly forming a relief pattern on a printing element using laser, which does not require the developing treatment, there can be mentioned a method in which a printing element is engraved directly with laser. Such a method is already used for producing relief plates and stamps, in which various materials are used for forming the printing elements.
In the following specific examples of the above-mentioned method for producing a printing plate (by directly forming a relief pattern on a printing element using laser), the term “printing element” means an element prior to the engraving with laser, and the “printing plate” means a plate obtained by engraving the printing element with laser to form a relief pattern on the printing element.
Examined Japanese Patent Publication No. Sho 47-5121 (corresponding to U.S. Pat. No. 3,549,733) discloses the use of a polyoxymethylene or polychloral for producing a printing element. Further, Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. Hei 10-512823 (corresponding to DE 19625749 A) describes the use of a silicone polymer or silicone fluoropolymer for producing a printing element. In each of the specific examples of compositions used for producing the printing element, which are described in this patent document, fillers, such as amorphous silica, are added to the above-mentioned polymer. However, a photosensitive resin is not used in the inventions disclosed in the above-mentioned patent documents. In the above-mentioned Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. Hei 10-512823, amorphous silica is added to the polymer for enhancing the mechanical properties of the polymer and reducing the amount of an expensive elastomer used in the printing element. Further, this patent document has no description about the morphology of the amorphous silica used.
Unexamined Japanese Patent Application Laid-Open Specification No. 2001-121833 (corresponding to EP 1080883 A) describes the use of a mixture of a silicone rubber and carbon black for producing a printing element, wherein the carbon black is used as a laser beam absorber. However, a photosensitive resin is not used in this invention.
Unexamined Japanese Patent Application Laid-Open Specification No. 2001-328365 discloses the use of a graft-copolymer as a material for producing a printing element. Further, this patent document describes that, for improving the mechanical properties of the graft copolymer, inorganic silica having a particle diameter which is smaller than the wavelength of the visible light may be mixed with the graft copolymer. However, this patent document has no description about the removal of liquid debris which is generated by laser engraving. Further, in the Working Examples of this patent document, a sheet of a photosensitive resin was formed from a liquefied photosensitive resin which had been prepared by adding a solvent to the resin; however, the production of such a sheet is disadvantageous in that it was necessary to remove the solvent by drying. In addition, the production of the above-mentioned sheet has drawbacks in that the working environment which is appropriate for the use of a solvent must be maintained and that the complete removal of solvent from the inside of the produced printing element is difficult when the thickness of the printing element is in the order of several millimeters.
Unexamined Japanese Patent Application Laid-Open Specification No. 2002-3665 uses an elastomer composed mainly of ethylene monomer units, and this patent document describes that silica may be added to the elastomer as a reinforcing agent. In the Working Examples of this patent document, 50 parts by weight of porous silica and 50 parts by weight of calcium carbonate were added to 100 parts by weight of a resin. Both of the above-mentioned porous silica and calcium carbonate were used only as white reinforcing agents and, for achieving a satisfactory reinforcing effect, those reinforcing agents were used in large amounts (total amount of the reinforcing agents was as large as 100 parts by weight). That is, the use of silica in this patent document does not extend beyond the customary use as a reinforcing agent. Further, the resin used in this patent document is not a photosensitive resin and the resin is cured by heating. Therefore, the curing rate of the resin is low and the dimensional precision of a sheet obtained from the resin is poor.
DE 19918363 A discloses an invention in which a printing element is produced from a polymer produced from recycled materials. This patent document describes not only a heat curable resin, but also a photosensitive resin. In the single Example of this patent document, a resin composition containing a heat curable resin and carbon black is used. The addition of even a small amount of carbon black to a resin leads to a lowering of the light transmittance of the resin. Thus, it is impossible to cure a resin composition (such as produced in the single Example of DE 19918363 A) containing more than 1% by weight of carbon black from the outer portion thereof through the inner portion thereof. Therefore, such a resin composition is not suitable for use in the production of a laser engravable printing element. The lowering of the curability of a resin by the addition of carbon black markedly occurs especially when a liquid photosensitive resin is used. Further, this patent document has no description about either an inorganic porous material other than carbon black or the removal of liquid debris generated by laser engraving.
Each of Japanese Patent No. 2846954 (corresponding to U.S. Pat. No. 5,798,202) and Japanese Patent No. 2846955 (corresponding to U.S. Pat. No. 5,804,353) discloses the use of a reinforced elastomer material obtained by mechanically, photochemically and thermochemically reinforcing a thermoplastic elastomer, such as SBS (polystyrene-polybutadiene-polystyrene), SIS (polystyrene-polyisoprene-polystyrene) and SEBS (poly-styrene-polyethylene/polybutadiene-polystyrene). When a printing element formed from a thermoplastic elastomer is engraved with a laser beam having an oscillation wavelength within the infrared region, even portions of the printing element which are distant from the portion irradiated with the laser beam also tend to be melted by heat. Therefore, the resultant printing element cannot be used for preparing an engraved pattern having a high resolution. For removing this problem, it is necessary to add a filler to the thermoplastic elastomer to thereby enhance the mechanical properties thereof. In each of the above-mentioned patent documents, for enhancing the mechanical properties of the thermoplastic elastomer and improving the absorption of the laser beam by the thermoplastic elastomer, carbon black having excellent ability to enhance the mechanical properties of a resin is added to a thermoplastic elastomer. However, since carbon black is added to the elastomer, light transmittance of the elastomer is lowered, which is disadvantageous when it is attempted to crosslink the elastomer by irradiation (i.e., when it is attempted to perform a photochemical reinforcement of the elastomer). Therefore, when the above-mentioned reinforced elastomer material is subjected to laser engraving, it results in a generation of a large amount of debris (including viscous liquid material) which is difficult to remove. The generation of such debris not only necessitates a time-consuming treatment for removing the debris, but also causes problems, such as the imprecise boundary between elastomer portions which have been melted by laser beam irradiation and unmolten elastomer portions which form the relief pattern, the swelling of the edges of the unmolten elastomer portions forming the relief pattern, the adherence of the molten elastomer to the surfaces and/or sides of the unmolten elastomer portions forming the relief pattern, and the destruction of portions of the relief pattern which correspond to the dots of a print obtained using the relief pattern.
Further, when a large amount of liquid debris, which is presumed to be a laser decomposition product of the resin, is generated during the laser engraving of the printing element, the liquid debris stains the optical parts of a laser engraving apparatus. When the liquid debris is attached to the surface of optical parts, such as a lens and a mirror, the resin causes serious troubles of the apparatus, such as burnout of the apparatus.
As apparent from the above, various materials for laser engraving have been proposed. However, a material for forming a laser engravable printing element, which not only has high uniformity in thickness and high dimensional precision, but also enables easy laser engraving without suffering problems caused by the generation of debris, has not yet been realized.